Internal combustion engines combust an air and fuel mixture within cylinders to generate drive torque. More specifically, air is drawn into the engine through a throttle and fuel is provided to the engine from a fuel system. The air and fuel are mixed at a desired air-to-fuel (A/F) ratio and is combusted within a cylinder to rotatably drive a crankshaft.
Some fuel systems include a fuel vapor purge valve to provide an evaporative emissions control. The purge valve is selectively actuated to deliver vapor fuel from the fuel system to be combusted within the engine. Many current production implementations of purge valve control use a fixed pulse-width modulated (PWM) frequency (e.g., 16 Hz).
Problems occur if the engine cylinder firing frequency becomes synchronized with the PWM purge frequency. For example, at an engine speed of 1920 RPM, one complete firing cycle (i.e., all cylinders fired) includes a period of 62.5 ms. For a PWM frequency of 16 Hertz, the fuel purge period is also 62.5 ms. Therefore, at 1920 RPM, the purge frequency is synchronized with the firing frequency of the engine cylinders. As a result, the purge fuel flow is delivered to the same cylinder or is possibly consistently split between a few cylinders. An A/F ratio imbalance is generated between the cylinders receiving the purge fuel flow and those not receiving the purge fuel flow, which can be detrimental to emissions, engine smoothness and driveability.